I want to make the cake shown in the sketch, using three 7" tiers as the base, then stacking on top of that an 12 (or maybe even an 11), an 8 and a 6.
What I noticed in some other designs was that there was a cake in the centre. Will I be able to stack the 12" on the edges of the 7"s as shown also in the sketch, or should my design include a fourth 7" to be placed in the middle of the three to give sufficient support?[/img]

You really should have the support in the middle, even if it's a styro dummy.

Hmmm, that's a great cost effective solution. Thank you. I will see what I can source.